Hydride-Free Hydrogenation: Unraveling the Mechanism of Electrocatalytic Alkyne Semihydrogenation by Nickel–Bipyridine Complexes

Hydrogenation reactions of carbon–carbon unsaturated bonds are central in synthetic chemistry. Efficient catalysis of these reactions classically recourses to heterogeneous or homogeneous transition-metal species. Whether thermal or electrochemical, C–C multiple bond catalytic hydrogenations commonly involve metal hydrides as key intermediates. Here, we report that the electrocatalytic alkyne semihydrogenation by molecular Ni bipyridine complexes proceeds without the mediation of a hydride intermediate. Through a combined experimental and theoretical investigation, we disclose a mechanism that primarily involves a nickelacyclopropene resting state upon alkyne binding to a low-valent Ni(0) species. A following sequence of protonation and electron transfer steps via Ni(II) and Ni(I) vinyl intermediates then leads to olefin release in an overall ECEC-type pattern as the most favored pathway. Our results also evidence that pathways involving hydride intermediates are strongly disfavored, which in turn promotes high semihydrogenation selectivity by avoiding competing hydrogen evolution. While bypassing catalytically competent hydrides, this type of mechanism still retains inner-metal-sphere characteristics with the formation of organometallic intermediates, often essential to control regio- or stereoselectivity. We think that this approach to electrocatalytic reductions of unsaturated organic groups can open new paradigms for hydrogenation or hydroelementation reactions.

analyzed by gas chromatography (GC) with the samples before and after the reaction. In addition, the cathodic chamber was sampled (150 µL aliquots) prior to and at the end of the electrolysis, the aliquots mixed with 250 µL deuterated chloroform and analyzed by 1 H nuclear magnetic resonance (NMR).

Analytical methods
Samples were analyzed by gas chromatography using a gas chromatograph equipped with a flame ionization detector (GC-FID; Nexis GC-2030, Shimadzu, Japan) with elution over a Rtx-1 column (30 m × 0.25 mm with 0.5 μm film thickness, Restek Corp., USA) using He as a carrier gas and a gas chromatograph equipped with a mass spectrometer (GC-MS; QP2020 NX, Shimadzu, Japan) with elution over a Rtx-1 column (30 m × 0.25 mm with 0.5 μm film thickness, Restek Corp., USA) using He as a carrier gas.
Integrals of the GC-FID peaks of the substrates and products were normalized over the one of the internal standard (mesitylene) for quantification. The quantification of carbon balance, alkyne conversion, alkenes yield, faradaic efficiency (FE) towards alkenes and turnover numbers (TONs) were calculated using the following equations: Yield (%) = conversion × selectivity (4) Where C i (S), C t (S), C i (S ) and C t (S ) are concentrations in alkyne S or alkene SH2 at the beginning of reaction (Ci) and at the given time (Ct), # (S ) is the amount of alkene at a given time, $ (Ni) is the amount of Ni at the beginning of the reaction, Qt is the charge passed through the system at a given time and F is the Faraday constant (96485 C·mol -1 ).
Headspace analyses were performed in a custom two-compartment H-type glass cell that differs from SI section 2.  Figure S1. CV (oxidation first) of 3. Figure S2. CVs of 1 (solid) or 4 (dashed) alone (black) or with S1 (10 equiv) (blue) or with S1 (10 equiv) and BzOH (50 equiv) (red). Potential (V vs Fc +/0 ) Figure S2 shows the comparisons of CVs for the two precatalysts 1 and 4. The reoxidation behavior (Ep,a = -1. 19 VFc) in the presence of S1 and in the catalytic conditions (S1 and BzOH) are fully consistent with the nickelacyclopropene resting state 3 as described in the main text. Interestingly, a negative shift of the foot of the reduction wave of ca. 50 mV is observed when switching from 1 alone (or with S1) to the catalytic conditions (1, S1 and BzOH). We propose that this behavior arises from the initiation required for 1 to enter the catalytic cycle (from a tris-bipyridine to a monobipyridine ligand set).

Comparison of redox behaviour of 1 and 4
Scheme S1 . Reduction events associated with 4 II/0 . Figure S3. CVs of 4 alone (black) or with BzOH (50 equiv) (orange) or with S1 (10 equiv) and BzOH (50 equiv) (blue). Figure S3 shows the CV of 4 upon addition of equivalents of BzOH (with or without S1 for comparison). The buildup of a minor electrocatalytic wave in the presence of BzOH only is ascribed to hydrogen evolution, as identified in an electrolysis experiment.

Behaviour of 4 in presence of benzoate
The voltammetry of 4 displays an ill-defined, broad reduction wave of Ep,c = -1.76 VFc, with a shoulder at ca. -1.65 VFc. We thus here aim at clarifying the reduction behaviour of 4.

S9
First, CV of 4 with scan reversal right at the top of the shoulder (Erev = -1. 67 VFc), which we designate as the first wave in the following, results in an irreversible wave ( Figure S4a). This behavior is suggestive that this first wave could subscribe to an electrochemical reduction (E) followed by a fast chemical step (C), in an EC fashion. DFT calculations (see SI section 5.4.1) also indicate that the one-electron reduction product of 4 [Ni(bpy)(BzO)2] -(I13) favors benzoate (BzO -) expulsion to yield [Ni(bpy)(BzO)] (I7). In reason of the irreversibility of the first wave, we disfavor a CE mechanism, as such mechanism would in general display a reoxidation wave. We thus tested the response of this wave towards BzOby incrementing concentration of this anion. We used for that DMF solutions of the nBu4NBzO salt, which features the same cation nBu4N + as our supporting electrolyte (nBu4NPF6) and so that we can exclude additional cationic effects.
First, we observe on the CV of 4 that upon increase in [BzO -] (≤ 5 mM) the first wave shifts negatively ( Figure S4b,c).
The influence of BzOon the first wave confirms that this electrochemical event is coupled to a chemical step Increasing further [BzO -] (≥ 5 mM) the first reductive event vanishes under the second one in a wave having maximized peak intensity (ip,c, ip,a) magnitudes. The apparent E1/2 (= (Ep,c+Ep,a)/2) of the resulting wave also displays a cathodic shift with increasing [BzO -] in a Nernstian fashion ( Figure S4d).
From this body of data, we propose that, with no added BzO - (Figure S5a), the second-order equilibrium between I13 to I7 is strongly displaced towards I7, with little influence of the backward association of BzO -. In this case, the first wave thus likely corresponds to the reduction of 4 to I13, followed by fast release of BzOto yield I7. Such a reduction scheme would subscribe to an EC mechanism, typically in the KP region of the kinetic zone diagram, 3 rather than a CE one. We also note that, at the potential of this first wave (Ep,c ≈ -1.65 VFc, with no added BzO -) the resulting complex I7 is likely not further reducible (into I9) and can only be reduced at more negative potentials, reached at the second wave. At the potential of the second wave (Ep,c = -1.78 VFc, with no added BzO -) I7 is reducible, leading to a second reduction event into I9. The reduction of I9 is likely reversible on the CV timescale, which would corroborate the apparent reversibility of this wave. We thus propose that the ill-defined nature of the overall reduction pattern for 4 actually arises from two reductive events of close redox potentials, at low [BzO -]. An electrolysis of 4 (alone) negative to these waves (Eapp = -2.1 VFc; data not shown) saturates current passed 2 mol(e -)/mol (4), which also confirms the 2-electron stoichiometry of the event. At high BzOconcentration (≥ 5 mM), we surmise that the influence of the backward association in the equilibrium between I13 and I7 cannot be neglected anymore ( Figure S5b). There, 4 is plausibly reduced into an equilibrated mixture of I13 and I7 (assuming that both forward and backward rate constants are fast at voltammetric timescale).
Under these conditions, the apparent redox potential of 4 would then be dictated by the equilibrium concentrations of I13 and I7. At this apparent potential, further reduction into I9 is likely accessible, giving rise to a better-defined twoelectron reduction wave experiencing a Nernstian shift with [BzO -]. We cannot fully exclude that, at this apparent potential, reduction of I13 (followed by BzOrelease) also becomes accessible.
We note that these interpretations of experimental data fall in good qualitative agreement with DFT results, which suggest that the reduction potentials of 4 and I7 (E 0 = -1.59 and -1.76 VFc, respectively) are close but not inverted. In addition, DFT data (see Scheme S2) also corroborates that the equilibrium between I13, BzOand I7 is moderately displaced in favor of the latter (by -2.8 kcal.mol -1 ) and accessible (∆∆G ‡ (TSI13-I7) = 4.5 kcal.mol -1 ), which, within

Additional mechanistic details
We discuss here the various plausible mechanistic pathways along the electrocatalytic wave and the influence of BzOon the electrocatalytic behaviour.
First, we stress that, under our experimental conditions, full conversion of the alkyne (10 mM) results in an expected BzOconcentration of 20 mM. Thus, concentrations relevant for catalysis are 0 ≤ [BzO -] ≤ 20 mM. We more particularly focus our discussion to the cases with no to low concentration of added BzOas our kinetic analysis (viz. FOWA) is based on initial concentrations.
In the case where no BzOis added, we observe that the addition of the alkyne S1 (10 mM) results in a positive potential shift of the first wave ( Figure S6a). This shift is indicative of an EC mechanism involving S1, viz. the fast and irreversible formation of I8, as proposed in our main manuscript (Scheme 4). This point is further confirmed by scan reversal at the top of this wave, which evidences in the backward anodic scan the oxidation wave of the nickelacyclopropene I8. Upon gradual addition of BzOH, catalysis develops from this wave ( Figure S6b). We thus attribute the activity on this wave to the mechanism initiating via I7, then I8, and following the ECEC-type pattern as described in our main manuscript.
When BzOis purposely added, the first electrocatalytic wave shifts towards more negative potentials (as observed for 4 alone), while the catalytic current at the potential of the second wave (E ≈ -1.78 VFc) appears less affected ( Figure S6c). At these more negative potentials, typically between -1.7 to -1.8 VFc, a pathway shuttling via the reduction of I7 is also plausibly accessible. This pathway may not be the one dominating at the early stage of catalysis, where the one shuttling via I8 is more favored. However, at late stage of catalysis when BzOconcentration has built up in the cell, the pathway via I8 likely shuts down and the one involving reduction of I7 can become predominant.
We thus tentatively assign the activity in this region of potential to the pathway for which I7 is reduced to I9, then leading to substitution of BzOby S1 into 3, from which follows the catalytic cycle described in our manuscript.
We used the CV data recorded at [BzO -] = 10 mM, which simulates a state of 50% conversion, to estimate kinetics when the catalytic cycle is shuttling via the reduction of I9 to 3 (on the second wave). Applying FOWA (taking E 0 = E(I7/I9) = -1.76 VFc from the DFT value estimate), we obtained an estimate of the TOFmax at 4.75 10 5 s -1 and a span of ca. 9.8 kcal.mol -1 . These values are also within good agreement with the values expected from DFT for the ECECtype mechanism described in our main manuscript. Finally, the wave observed under electrocatalytic conditions at Ep,c ≈ -1.95 VFc can likely be attributed to another mechanism, possibly implying the reduction of I13 or the generation of a nickel hydride.
Overall, we propose that the electrocatalytic waves represent three main regimes, as schematically pictured on Figure   S6d and summarized in Figure S7. At early stages of catalysis and potentials close to the foot-of-the-wave ( Figure   S6d, pale blue area), the pathway shuttling via I8 (described in the main manuscript) is predominant. At advanced conversion leading to substantial [BzO -] in the cell and for intermediate potential values (ca. -1.7 to -1.8 VFc; Figure   S6d, dark blue area), a pathway involving I9 as intermediate to 3 is accessible and joins the catalytic cycle described in Scheme 4. At more negative potentials (< -1.8 VFc; Figure S6d, red area) another mechanism, for instance via the reduction of I13 or a hydride, is also competing. The degradation of the FE toward alkene at more negative applied potentials along the electrocatalytic wave ( Figure S8) supports that the ECEC-type mechanisms predominating close to the foot-of-the-wave have higher selectivity for alkyne semihydrogenation.

Assessment of electrocatalytically active deposits
The assessment of heterogeneous deposits responsible for the activity in electrocatalytic semihydrogenation was largely addressed in our previous manuscript 4 (especially in SI section 3.2.3.6). These results all suggest that, while we cannot fully rule out the deposition of (small) Ni species on the electrode, such species are minor and not

Develops at first wave
More positive than potential of first wave We reproduced here the "non-rinse" test with 4 since this test is a good reporter to evaluate the activity of any deposited species, whatever the nature of these species. We found that the current ( Figure S9.) and activity (yield in S1H2: 5.9 % vs 86.4 % for the first run including 4) of the post-electrolysis, non-rinsed electrode are minor, which further supports that no catalytically active deposits form during electrolysis with 4. In addition, we also performed an electrolysis using a Ni foam (purchased from Goodfellow; 1.6 mm thickness; 95% porosity) under conditions identical to with 4 ( Figure S10). Under these conditions, alkyne consumption is below traces, whereas the use of 4 (at a glassy carbon foam) results in full conversion. These results underline that a Ni foam, which was reported active under other conditions, is not an active electrocatalyst under our conditions. Interestingly, we also observed that the addition of the molecular pre-catalyst 1 in the electrochemical cell during electrolysis with Ni foam restores the activity. This result further proves the requirement of such molecularly-defined species for catalysis. The tolerance of the system for various electrode surface (C foam; Ni foam; Ag foil, data not shown) is also an additional indication that catalysis is driven by molecular species.

Synthesis of [Ni(bpy)2] (2)
[Ni(COD)2] (137.5 mg, 0.50 mmol, 1.00 equiv) and 2,2-bipyridine (156.2 mg, 1.00 mmol, 2.00 equiv) were introduced in a Schlenk tube in the glovebox and 5 mL of THF were added. The resulting deep blue solution was stirred 2 h at room temperature. The solvent and COD were evaporated under prolonged exposure to vacuum (10 -3 mbar). The obtained solid was dissolved in THF (2 mL) and layered with pentane (10 mL). After 18 h, the solution was carefully removed from the solid and washed with pentane (3 x 10 mL) and dried under high vacuum (10 -3 mbar) to afford 97 mg of 2 as dark shiny plates (yield = 52%).
The NMR data ( 1 H, 13 C; Figure S11, Figure S12) are in agreement with the literature.

Synthesis of [Ni(bpy)(PhCCMe)] (3)
The procedure is inspired from the reported synthesis of [Ni(bpy)(PhCCPh)]. 2 [Ni(COD)2] (137.5 mg, 0.50 mmol, 1.00 equiv) and 2,2-bipyridine (78.1 mg, 0.50 mmol, 1.00 equiv) were introduced in a Schlenk tube in the glovebox and 5 mL of THF were added. The resulting deep blue solution was stirred 2 h at room temperature and then kept in the freezer for at least 30 min at -30 °C. 1-ph-1-propyne (S1) is then added dropwise (62.6 µL, 0.55 mmol, 1.1 equiv) and the resulting red brownish solution was warmed up to room temperature and stirred overnight. This solution was then filtered over glass frit (P4 pore size) in glovebox and the solvent was evaporated under vacuum. The dark powder obtained was dissolved in THF (0.5 mL), layered with pentane (3 mL) and kept at -30 °C for a few hours. Small dark plates suitable for XRD were obtained. The solution was then carefully removed from the solid that was then washed with pentane (3 x 5 mL) and dried under vacuum (10 -3 mbar) to afford 107 mg of 3 as a dark solid (yield = 65%). This compound is highly sensitive and degrades in a J. Young NMR tube after 12 h.

Synthesis of [Ni(bpy)(BzO)2] (4)
[Ni(bpy)(PhCCMe)] (3) (66.0 mg, 0.20 mmol, 1.00 equiv) and benzoic acid (48.8 mg, 0.40 mmol, 2.00 equiv) were introduced in a Schlenk tube in the glovebox and 5 mL of THF were added. The resulting grey blue-ish suspension was stirred 1 h at room temperature. The solvent was evaporated under vacuum and the obtained solid washed with THF (5 mL) and pentane (2 x 10 mL) and dried under high vacuum (10 -3 mbar) to afford 58 mg of 4 as light blue powder (yield = 63%). This solid is poorly soluble even in DMSO or DMF. However, we found that it is soluble in the presence of nBu4NPF6 in DMF (up to 4 mM, with a very slow solubilization which can be faster by using sonication or heat).
The 1 H NMR spectrum shows the appearance of two quadruplet signals at 5.0 and 5.3 ppm (J = 6.5 and 6.7 Hz respectively) in ca. 15% yield ( Figure S17), which we attribute to vinylic protons, as these signals are close to the ones observed for a reported nickel(II) vinyl complex. 8 The absence of singlet signals suggest that the product of protonation in a-phenyl position is not formed and that the protonation is thus a-methyl regioselective. The two quadruplets are attributed to the two stereoisomers I2 and I2E in a ratio of 8:2. The methyl protons of I2 were also observed, at δ( 1 H) = 2.8 ppm (J = 6.7 Hz). Despite repeated attempts, we were not able to isolate these species for further characterization. In the absence of nBu4NPF6, these signals were not observed and only the alkene products were obtained (see below).
The 1 H NMR spectrum (recorded after few hours to let the precipitate sediment, Figure S18
Young NMR tube in the glovebox at r.t. and mesitylene was added as the standard (7.0 µL, 50 µmol). A grey blue-ish precipitate is directly obtained that corresponds to 4 (checked by 1 H NMR in DMSO-d6, see above SI section 3.3).
The 1 H NMR spectrum shows the formation of H2 in ca. 70% yield ( Figure S19, Figure S20).  We note that the wide range 1 H NMR spectrum of the mixture of 2 with one equivalent of BzOH does not display signals characteristic of a hydride species at Ni ( Figure S20).

[Ni(bpy)(PhCCPh)] (5) + BzOH
[Ni(bpy)(PhCCPh)] (5) (3.9 mg, 10 µmol) and nBu4NPF6 (38 mg, 100 µmol) were dissolved in 0.5 mL of DMF-d7 in a J. Young NMR tube in the glovebox and kept for at least 30 min at -30 °C. BzOH (1.2 mg, 10 µmol, 1.0 equiv) was added and the mixture was allowed to warm up to r.t. (Figure S21). The 1 H NMR spectrum shows the appearance of a singlet at 6.01 ppm ( Figure S21), which we tentatively assign to the corresponding [Ni(bpy)(PhCCHPh)(BzO)] vinyl intermediate. In that case, only one isomer is detected, which is likely the Ni-cis-protonated one, by comparison with the shift of the free (Z)-olefin and in agreement with the shift reported for a similar stilbene Ni(II)-vinyl complex. 8 Interestingly, the 1 H NMR also reveals the evolution of the two isomers of the olefin product in a Z/E 59:41 ratio. Based on these results, we surmise that both vinyl isomers are accessible, but only the (Z) one is stable enough for trapping and observation on the experimental timescale, while the (E) one is less stable and readily converts to the corresponding olefin. Such a trend in relative stability of the vinyl isomers is also evidenced by computation on the Ni-vinyl isomers evolved using the 1-phenyl-1-propyne S1 substrate (see Scheme 4 in main manuscript).
In the absence of BzOH, the 1 H NMR spectrum namely shows two broad singlets at 10.14 and 10.08 ppm ( Figure   S22), in very good match with the ones observed for chemically synthesized 3 (see above SI section 3.2). In the presence of BzOH (1 or 2 equiv) the 1 H NMR spectra show signals of the S1H2 alkene in a Z/E ratio of 8:2 ( Figure   S23). When only one equivalent of BzOH is present, a quartet at 5.04 ppm (J = 6.5 Hz) can additionally be observed ( Figure S24) and that we attribute to the Ni(II) vinyl complex I2 (see SI section 3.4.2).

Foot-of-the-wave (FOWA) analysis
Kinetic data in this work are based on the foot-of-the-wave (FOWA) analysis developed by Savéant and coworkers 9 that enables extraction of the maximum turnover frequency (TOFMAX) where the CV response is not affected by side phenomena. The proposed catalytic cycle is based on a mechanism of the type ECEC, as supported by DFT calculations (Scheme 4, main text) and for which equation (7) can be applied, but which is not restricted to this case (R is the perfect gas constant, T = 298 K and n is the scan rate). ip and E° correspond to the peak current of a oneelectron wave and the apparent half-wave potential of the catalyst 4 and were obtained from the CV of the catalyst in the absence of substrates.
We note that the assessment of the first reduction potential of 4 is obtained from an ill-defined experimental wave, which thus leads to uncertainties in the estimation of this value (E 0 = -1.64 VFc, at the experimental first wave), and that at high concentration of benzoic acid, homoconjugation phenomena 10 can impact the electrocatalytic answer. For FOWA at high [BzO -], E 0 = -1.76 VFc was assumed from the DFT estimate of E(I7/I9).
We also point that the reduction of 4 is associated with an initiation process that only intervenes in the first catalytic cycle (Scheme 3Ab or Scheme S2). In general, the interplay of the initiation in the electrocatalytic wave complicates the electrochemical analysis. However, the initiation process pertains to initial catalytic conditions, at which the electrocatalytic wave develops and hence for which FOWA was applied.
The analysis of the values of TOFMAX with varying concentrations of the different reaction partners allows to extract partial orders in each partner. The kinetic isotope effect, KIE, was determined by evaluating TOFMAX for the reaction in the presence of the deuterated acid BzOD.
For higher accuracy, the currents of the CVs were normalized by subtracting the current from the blank CV performed at the beginning of each experiment. Kinetic analyses for alkyne, BzOH and BzOD (KIE) were also replicated with at least two different scan rates (0.05, 0.1 or 1 V.s -1 ) to validate applicability and to provide uncertainty values.
S30 Figure S25. Representative example of data treatment for FOWA analysis.

General consideration
Calculations were performed using the ORCA 5.0 suite of software. 11 The PBE0 functional 12 with Grimme's D3BJ dispersion correction 13 was used in conjunction with the 6-311+G(d,p) basis set for all atoms. 14-18 All geometries were fully optimized without any symmetry or geometry constraints. Harmonic vibrational analyses were performed to confirm and characterize the structures as minima or transition states. Free energies were calculated within the harmonic approximation for vibrational frequencies. The effects of the solvation by DMF were included in the energy calculations using the CPCM model. 19 Standard potentials were calculated with respect to the phenazine 0/redox couple and converted back versus the Fc +/0 redox couple as previously described for accuracy. 20 The electron transfer steps were then calculated at -1.70 V relative to Fc +/0 and are assumed to be faster than chemical steps. Nevertheless, the UKS function implemented in ORCA was used for spin-unrestricted optimisation.

Computed EECC pathway
Following the initial protonation step (C) of the mechanism presented in the main text, I2 (-5.5 kcal.mol -1 ) can be obtained. A second protonation step (C) on I2 most likely proceeds in an outer-sphere manner (see SI section 5.4.3 for inner-sphere) with a relatively high energy barrier involving TSI2-I12 (+15.6 kcal.mol -1 ) and gives the cationic species I12 (-3.5 kcal.mol -1 ) (Scheme S2). From I12, the (Z)-alkene product is readily released via However, in all these cases, the energetic span of the reaction is defined by I2 and TSI2-I12, which corresponds to the second outer-sphere protonation step, and is found at a value of 21.1 kcal.mol -1 . This value is much higher than the span of 10.2 kcal.mol -1 associated to the ECEC-type mechanism (Scheme 4 in the main text). For that reason, we propose that, under electrocatalytic conditions, the ECEC-type mechanism is largely dominant over the EECC one.
Several other pathways were ruled out due to high energy values for intermediates or TSs, or reduction potentials of intermediates more negative than the applied potential of ca. -1.7 VFc (see SI section 5.4.3).

Hydride pathways
Scheme S3 represents other nickel hydrides that have been considered in the DFT calculations. In particular, the reduced form of I10, I14 which is more stable. However, the transition states TSI14-I3 and TSI14-I7 corresponding to either hydrometallation or hydrogen evolution from I14 display relatively high barriers for these two elementary steps (DDG ‡ = +25.2 and +17.3 kcal.mol -1 respectively).
Despite repeated attempts to generate hydrido-vinyl Ni species, the only stable one that converged is the cationic Ni(IV) complex I15, which is though +32.9 kcal.mol -1 higher in energy than 3. The corresponding Ni(III) hydride complex, which can be obtained by protonation of I3, was not found because the structure converges to I6 during optimization.

Stereoisomers, regioisomers and alternative transition states
Different pathways were studied to understand the stereoselectivity of the reaction (Scheme S4). From 3, only the TS3-I1 transition state leading to I1 was found. A possible TS leading to I1E in one step could not be found because during optimization, the geometry of TS3-I1 was always restored. After electron transfer, two species also in equilibrium I4 and I4E are obtained.
The (E) isomers of the vinyl species I2 and I3, i.e., I2E and I3E, were found to be thermodynamically more stable than the (Z) isomers, which does not reflect the observed stereoselectivity among olefin products. Furthermore, (Z)to-(E) isomerization at I2 is very unlikely, due to a very high transition state energy of TSI2-I2E (DG ‡ = +32.5 kcal.mol -1 ). These latter selectivity-determining pathways were therefore excluded.

Three lowest frequencies and Gibbs free energy for all computed structures
Structure geometries can be found in the xyz document attached.